Fig. 2WARMER projections of marsh elevation under three SLR scenarios.

We incorporated low, moderate, and high SLR rates (6) into the WARMER model to project 2010–2110 change in mean marsh elevation. Under a low SLR scenario, many tidal marshes were projected to lose little elevation through most of the century. However, under higher rates of SLR, most wetland study sites lost mean elevation and did not “keep pace” with relative SLR, resulting in the loss of wetland elevation. MHHW is at z* = 1.0; mean tide level (MTL) is at z* = 0.0.

Fig. 3Habitat projections from WARMER modeling under three SLR scenarios.

Under moderate and high SLR scenarios, all study sites are projected to undergo substantial loss of elevation over the coming century, resulting in major changes in the composition of tidal wetland habitat types. By 2050, under moderate and high SLR scenarios, there is a gradual loss of high marsh habitats with an expansion of middle and low marsh habitats. Under moderate SLR scenarios by 2110, there is a loss of middle and high marsh habitats and submergence of tidal marsh, with a conversion to intertidal mudflat and open water at 36% of our study sites. Under high SLR scenarios, there is a total loss of all middle and high marsh habitats and submergence at 86% of the study sites, with three study sites going partly subtidal.

We assessed relative differences in overall wetland vulnerability across the Pacific coast by pairing available upland migration space with modeled vertical wetland elevation change under high SLR using WARMER. Migration potential index was calculated by dividing the current marsh area in the estuary by the area of suitable upland migration area. California sites were the most vulnerable because of substantial wetland elevation loss and minimal migration potential under a high SLR scenario. No study sites had enough low-elevation adjacent upland to allow 1:1 replacement of the current wetland area, and most wetlands had less than 50% available land for replacement. WARMER ratio was calculated by dividing the ending 2110 elevation by the beginning 2010 elevation under a high SLR scenario. WARMER ratio represents a site’s ability to maintain elevation through time. Colors represent risk at 25% intervals, from blue shades illustrating highest risks of submergence to green shades suggesting lowest risk of submergence from SLR.

Fig. 5Modeling results illustrate changes from current habitat composition to greater extent of low marsh and mudflats under high rates of SLR at Newport Bay, which sits within the urban landscape of Southern California.

(A) Currently, there is a mix of high, middle, and low marsh that provides habitat to a variety of endemic threatened and endangered wetland species, but (B) middle and high marsh habitats are projected to be lost by 2050, decreasing plant community complexity and diversity and available habitat. (C) Increased availability of waterbird habitat may occur with expansion of subtidal and intertidal mudflats, but the complete loss of wetland vegetation is projected to occur by 2110.